The present invention relates to a personal authentication device using a living body, and in particular a living body personal authentication device based on a blood vessel pattern of a finger.
As a highly convenient security method that does not require to carry a key or the like and reduces the risk of illegal use due to loss or a theft, living body authentication using a part of a personal body, such as a fingerprint, iris, or a blood vessel pattern, as a key is attracting attention. Among them, an authentication method using a blood vessel pattern brings about little rejection symptom. Because the authentication method does not remind us of crime investigation unlike the fingerprint, and an eyeball is not directly exposed to light unlike the iris. Furthermore, since the blood vessel pattern is not a living body surface, which can be easily absorbed, but is an internal feature, the authentication method using the blood vessel pattern has an advantage that forgery is difficult.
Such a blood vessel pattern within a living body is obtained by illuminating a subject region with a infrared light source and capturing an image thereof with an image capture system, such as a camera or an image sensor, having sensitivity to infrared light. Since hemoglobin in blood absorbs infrared light well, light is absorbed in the blood vessel portion and the blood vessel portion appears dark in the image as compared with peripheral tissues. A pattern obtained by a difference between the brightness and darkness becomes the blood vessel pattern.
There are two methods for capturing an image of the blood vessel pattern. One of the two methods is a reflection method of capturing reflected light. The other of the two methods is a transmission method of capturing transmitted light obtained by illumination from the back. Such an authentication device is disclosed in, for example, JP-A-2002-083298.
In the reflection method, the light source and an image capture system can be disposed on the same side as one body. Thus, the reflection method has an advantage that the device is housed compactly and the space located across the finger from the device is not occupied, resulting in openness. In the case where reflected light is captured, however, light emitted from the light source is reflected by the epidermis not a little. The strength of light that arrives at the vicinity of the blood vessel located in the subcuteneous tissue, which is located under the epidermis, and is reflected becomes relatively weak. Therefore, the captured image lacks pattern clearness. As a result, it is difficult to obtain a pattern having repeatability, unless the blood vessel is clear to such a degree that visual recognition is possible even under the visible light, like a vein located in back of a hand having thin epidermis. This means that the pattern can be reproduced easily, and there is a serious problem as a security technique in the aspect of forgery resistance. In addition, because of the property that reflection of light from the epidermis is intense, the influence of a change of the skin surface, such as an injury, skin chapping, or wrinkles, on the captured image is inevitably great.
On the other hand, in the transmission method, there is a limit in the thickness of a living body through which light can be transmitted. However, light emitted from the light source and reflected by the epidermis is returned only to the light source side, and the image capture system is not affected. The finger has just a thickness through which light can be transmitted. In particular, as for the blood vessel pattern located on the palm side, the blood vessel itself is thin, and the epidermis is also thick as compared with the back. Even if an image of the blood vessel pattern is captured by a camera, therefore, the transmission method is excellent in forgery resistance.
For capturing the transmitted light, however, it is necessary to interpose the finger between the light source and the image capture system. If the light source part, the finger, and the camera part are arranged in a vertical straight line in the cited order as shown in FIG. 9, therefore, the light source part hides the finger as if it is a cover, when seen from the position of the eye at the time of operation. The authenticating user feels pressure because the user cannot see the finger. Furthermore, if an operation button or the like is disposed in the finger tip position, the user might waver in grasping the button position. On the other hand, if the light source part, the finger, and the camera part are arranged in a horizontal straight line as shown in FIG. 10, then there isn't anything, such as a cover, for hiding the top portion, and the finger can be seen from the viewpoint position. However, there is a problem that the hand that can be used for authentication is limited to the left hand or the right hand according to the position relation between the light source and the camera. FIG. 10 shows the case where the light source is disposed on the right side. If the hand is the right hand, then it is easy to turn the palm side of the finger toward the camera part. In the case of the left hand, however, it is necessary to forcibly wrench the arm to turn the palm side toward the camera part. In other words, in an authentication device having a camera part on the left side and a light source part on the right side, authentication other than that using the right hand is not practical. In the case of the opposite arrangement, authentication using the left hand is compelled. Of course, it is conceivable to prepare two sets each including a light source and a camera, symmetrically so as to be associated with the right hand and left hand, respectively. However, there are many problems, such as a cost increase and necessity of camera switching.
As a solution to the problems, a method shown in FIG. 12 is conceivable. In this method, the light source, the finger and the image capture system are arranged in a vertical direction, but they are not arranged simply in the vertical direction. Light sources are disposed obliquely above the finger in the tilted state. As a result, the finger is not hidden from the eye position at the time of operation and the feeling of pressure can be mitigated, as compared with the conventional vertical alignment method shown in FIG. 11. In addition, it is not necessary to limit the hand to be used for authentication to the left or right hand.
Even if the light sources are disposed obliquely above the finger, however, side walls for supporting the light sources having the conventional height are needed, and feeling of pressure is left not a little. In addition, the size of the entire device scarcely changes. Of course, if the side walls are made lower, then the feeling of pressure is eliminated. Even in this case, light emitted from the light source to side faces of the finger is scattered inside, and consequently the light also reaches the palm side of the finger, which is not on the straight line in the incidence direction. In other words, transmitted light exists on the palm side of the finger in the same way as the case where the light source is disposed on the back side of the finger. If the light sources are disposed in low positions, however, light strikes directly palm side portions and the light is reflected. For the same reason as that in the reflection method, it becomes difficult to obtain a blood vessel pattern from each of the palm side portions. Since the amount of light reflected by the side face of the finger is large, regions in which the brightness is saturated at its maximum value are formed in the captured image and parts of the blood vessel pattern are lost, as shown in FIG. 13 as a typical example. If the output strength of the light sources is adjusted, then the area of each of the saturated regions can be restrained to be small. In that case, however, the light amount reaching the vicinity of the center on the palm surface of the finger conversely becomes insufficient, resulting in a problem that the blood vessel pattern cannot be obtained. In other words, unless the ratio of the reflected light to the transmitted light is suitably adjusted, a correct blood vessel pattern is not obtained. If the side walls are low, then external light other than the light emitted from the light sources also strikes directly the side face of the finger sideward or obliquely. This becomes a cause of aggravation in the authentication performance under the strong external light, such as the setting sunlight.